The invention pertains to video signal processing devices for generating television synchronization signals for combining with the video signal being processed. More particularly, the invention relates to television synchronizing signal waveform generators in which the various synchronizing signals to be combined with the video signal are generated digitally.
A television signal is a composite of several different signal components, generally, falling within one of two classes of signals, namely, a video information signal component and several synchronizing signal components. The common television signals are formed of lines or horizontally distributed video information separated by intervals of horizontal line relates synchronizing signals defining the beginning of each line. The horizontal lines are further organized into rasters of vertically distributed lines defining fields of lines separated by vertical field related synchronizing signals. In turn, the fields are organized into frames, each composed of two interlaced fields of horizontal lines, with lines of each field having a different raster position upon display. The various synchronizing signals included in television signals serve to synchronize the processing of the television signals and the operation of the processing and other television signal utilization devices.
In color television signals, the synchronizing signals include vertical and horizontal blanking intervals, each formed of a composite of several synchronizing signals. The vertical blanking interval includes a vertical blanking level extending between leading and trailing signal transition edges that determine the durations of the vertical blanking interval. Onto this blanking level is impressed a number of horizontal blanking intervals, a number of equalization pulses, a serrated pulse interval defining a vertical sync pulse, and a burst (typically 9 to 11 cycles) of the sinusoidal chrominance subcarrier signal (color burst) following each horizontal sync pulse during about the latter one-half of the vertical interval. Each horizontal blanking interval during the latter one-half of the vertical blanking interval and the entire field of lines between consecutive vertical blanking intervals includes a horizontal blanking level extending between leading and trailing signal transition edges that determine the duration of the horizontal blanking interval. Impressed on each horizontal blanking level is a horizontal sync pulse followed by a color burst. One horizontal sync pulse and one color burst are provided for each horizontal lines of the television signal and serve to keep the horizontal scanning and color generation synchronized. The vertical sync pulse is provided for each field of the television signal to keep vertical scanning synchronized. The serrations of the vertical sync pulse prevent loss of horizontal scanning synchronization. Equalization pulses are provided to insure proper scanning motion synchronization with the required interlacing of the two fields that compose a television frame. The horizontal and vertical blanking levels serve to blank the display during horizontal and vertical retraces, with the associated transition edges effecting a smooth signal change between the video information signal intervals and the blanking intervals.
Proper display and processing of television signals requires precise formation of the synchronizing signals and insertion of them within the television signal. In the generations of television signals, the video information is usually generated separately from the synchronizing signals, with the two being added together, i.e., combined, in a multiplexer. Furthermore, during post-generation processing of television signals, new synchronizing signals are usually inserted in the processed television signals. This combining or insertion is performed at the conclusion of the generation or processing to avoid the introduction of timing disturbance to and the degradation of the synchronizing signals. Moreover, transmission of television signals through communication channels often introduces such disturbance and degradation. A video tape recorder (VTR) is an example of such a communication channel. Following such transmission, new synchronizing signals are inserted in the television signal to restore it to its proper form. Video processing amplifiers are commonly employed to insert the television synchronizing signals into the video information.
One particular important timing relationship between the various components of color television signals is the phase of the color burst relative to the horizontal sync pulse. The phase of the burst is commonly measured relative to the 50% point of the leading edge of the preceding horizontal sync pulse. If noise, signal transmission, or VTR operation distorts the synchronizing signals, it usually leads to incorrect processing and display of the television signal.
Such distortion often has the particularly undesirable effect of altering the phase of the synchronizing signals. This phase alteration complicates the processing of television signals, particularly, when provided by different television signal sources or when a television signal experiences several record and reproduce sequences creating multiple generations of the television signal. For example, distortions of the edges of the horizontal sync pulses can result in errors of measurement of the phase of the color burst, with different distortions producing different measurement errors. If several VTRs or other television signal sources are used to generate a program and the phase stability is not maintained between the several sources, different color burst phase measurement errors can result, leading to the inserting of color burst at a different phase relative to the edge of horizontal sync for signals received from different sources. If, for example, one VTR is used as a source of an entertainment program, and another VTR is used as the source of a commercial or bulletin, when a switch of sources is made, the phase of color burst relative to horizontal sync may suddenly shift because of the phase difference between the two sources. This can cause a sudden shift in the hue of the objects in the displayed television picture. Creating multiple generations of a television signal also can lead to such undesirable results, because each record and reproduce sequence often is accompanied by small distortion of the sharp signal transition edges, which can accumulate with each generations of the television signal and produce noticeable degradations in the display of such television signals.
Further, there exist national standards, such as NTSC RS170A standard, which exactly specify the time duration, rise time, edge shape, and time relation between the varous components of a composite video signal for public transportation, including very specific standards regarding the shapes and times of occurrence of the synchronizing signals contained in the composite television signals. Such precise standards must be met for proper function of video systems. This is the reason new synchronizing signals are locally generated and inserted into the video signal being processed in place of the original synchronizing signals.
In existing video processing devices, such as used in digital time base correctors, it is common to convert the digitized video to analog form before insertig the synchronizing signals. This process and architecture have several drawbacks, including crosstalk and phase drift and other forms of instability. Generally, the digitized video is converted to analog form and synchronizing signals are generated in filters for insertion at the proper location relative to the video information. However, the circuitry that handles the video information portion of the television signal generally is in close proximity to the synchronizing signal generating circuitry and signal lines in each circuit have a certain amount of inductive coupling to each other. Since relatively narrow pulse widths and sharp rise times are characteristic of the synchronizing signals, high frequency components are generated which can be radiated and picked up on the video information circuitry as crosstalk. Such crosstalk can cause undesirable disturbances in the displayed video information.
The problem of phase instability also arises in systems where the video information is digitized, and the inserted synchronizing signals are generated in analog form. Generally, the analog circuits used to generate the synchronizing signals are not locked in synchronization with the clock driving the digital video information processing circuitry. This lack of a locked synchronization relationship naturally leads to variations in the phase between the analog synchronizing signals and the digital video data.
Thus, a need exists for a system for digitally generating television synchronizing signals and combining them in synchronization with the video information so that a stable phase relationship is maintained between the synchronizing signals and the video information with which they are combined.
In accordance with the present invention, television synchronizing signals to be combined with television video information in a signal combiner are generated digitally by a digital number generator that provides digital signal values representative of the amplitude peaks of the synchronizing signals. For monochrome television signals, digital signal values are provided that represent the amplitude peaks of the blanking levels and the sync and equalizing pulses. If color television signals are formed, digital signal values also are provided that represent the amplitude peaks of the several cycles of color burst that follow the horizontal sync pulse. The times and intervals of the generation of the digital signal values are determined by a reference signal that identifies when the synchronizing signals are to be inserted in the video information signal. This reference signal is coupled to control the digital number generator so that it issues the appropriate digital signal values at the appropriate times for the appropriate duration. To assure the insertion of the synchronizing signals correctly within the video information signal, the reference signal also is employed to synchronize the transmission of the video information signal through its signal path preceding the signal combiner so that the arrivals of synchronizing signals and video information signal at the signal combiner are coordinated to effect the desired combining of the signals.
The generated digital signal values precisely define the amplitude peaks, but not the edges of the synchronizing signals. As described hereinbefore, the signal transition and other edges of the synchronizing signals are precisely specified for television signals used for public broadcast. In such signals, these edges are defined by complementary sine squared functions, one for rising edges and the other for falling edges. The sine squared function is given by the expression EQU y=(sin x).sup.2
where x has values from 0.degree. to 90.degree.. The complement is given by the expression EQU y=1-(sin x).sup.2
A particularly salient feature of the present invention involves the technique of processing the digital signal values provided by the first digital number generator to effect the shaping of the edges forming the synchronizing signals according to the sine squared functions. More particularly, a plurality of digital gain control values representative of a sine squared edge shape are generated by a second digital number generator to occur synchronously with the commencement and conclusion of each synchronizing interval. The times and intervals of the generation of the digital gain control values are determined by the aforementioned reference signal that identifieswhen the synchronizing signals are to occur in the video information signal. This reference signal is coupled to control the second digital number generator so that it issues the appropriate digital gain control values at the appropriate times for the appropriate duration. The generated gain control values are coupled to a first input of a digital multiplier. A second input of the multiplier is coupled to receive the digital signal values provided by the first mentioned digital number generator. In the multiplier, the digital signal values are multiplied by the digital gain control values and, thereby, adjust the digital signal values at the commencements and conclusions of the synchronizing signals according to the sine squared function represented by the digital gain control values.
In another embodiment, the digital signal values defining the peak amplitudes and the digital gain control values defining the shapes of the edges of the digitally synthesized synchronizing signals may be multiplied "in front" of the multiplexer. The multiplexer inputs would then be the video information signals on one channel and the already multiplied numbers representing the digitally synthesized synchronization signals on the other channel. The multiplexer is switched at appropriate times to place the digitally synthesized synchronization signals in the appropriate places in the composite television signal being generated.
For other than conventional television signals, the transition edges of the synchronizing signal may be defined by functions other than the sine squared function. For such other television signals, the digital gain control values are selected to effect the shaping of the transition edges according to the function or functions required for proper shaping of the synchronizing signals.
According to a particularly advantageous feature of the present invention, an addressable memory is employed to store digital gain control values from which the edges of the synchronizing signals are generated. The generation of the digital gain control values for a particular edge of a particular synchronizing signal is achieved through the control of the address generator that effects retrieval of the digital gain control values from the memory storage locations. While a separate set of gain control values could be stored and retrieved for each edge of each synchronizing signal to be combined with the video information, the preferred embodiment is featured by storing a single set of digital gain control values from which all edges required of al synchronizing signals for a particular television standard are generated. Moreover, the single set of stored digital gain control values are employed for forming the rising and falling edges defined by related, but complementary, sine square functions. Complementary digital gain control values for one kind of edge, for example, a falling edge, before providing them to the multiplexer, while providing uncomplemented values to the multiplier for the other, rising edge. This complementing is conveniently accomplished in the preferred embodiment by controlling the address generator to effect retrieving the gain control values in reverse sequences for the complemented and uncomplemented values. The use of an addressable memory in this manner greatly simplifies and facilitates the generation of the digital gain control values.
For analog television signal utilization devices, such as television signal display monitors, the composite digitized television signal is coupled to a digital to analog (D/A) converter. The D/A converter is operated to convert to analog signal from both the digitized video information signal component of the composite television signal and the digitized synchronizing signal components combined with the video information signal component to form the composite digitized television signal. A filter cooperates with the D/A converter to form the usual continuous composite analog television signal from the series of discrete analog amptitude values customarily provided by the converter. To simplify this formation of the analog television signal, a single, low pass filter is used, with the filter selected to have an upper corner frequency of a little less than two times the color subcarrier frequency and an upper stop band that rolls off to at least -6 decibels (dB) at a frequency of two times the color subcarrier frequency and to at least -55 dB at a frequency of three times the color subcarrier frequency. A single filter with the foregoing characteristics enables the smoothing of the entire composite television signal provided by the A/D converter in the form of a series of discrete amplitude values.
By generating the television synchronizing signals in the digital domain synchronously with the timing of the video information signal with which it is to be combined, combining the digitized synchronizing signals with the video information signal that is compatibly digitized to form a digitized composite television signal, and processing the composite television signal without separating the video information signal component from the synchronizing signal components while preparing the composite signal for use by a television signal utilization device, it is possible to provide television signals with precisely shaped synchronizing signals, while establishing and maintaining stable phase relationships between the various synchronizing signals themselves as well as between them and the associated video information signal.